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How to Measure Replacement Windows

On this page:
  • 3 steps for measuring windows
  • How to measure window frames
  • Finding a local window installer

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How to Measure Home Windows

Screen Shot 2018-05-08 at 4.30.46 PMStep 1. Take 3 horizontal measurements of the window frame for the rough opening width. 

  • Measure the distance horizontally across your frame at the top, at the middle and at the bottom from each side jamb.
  • Circle the smallest measurement of the three figures.
  • The smallest measurement is your rough opening width, the main measurement to reference when purchasing a replacement window.


* Tip: Always list window measurements with width before height.

Screen Shot 2018-05-08 at 4.32.20 PM

Step 2. Take 3 vertical measurements of the window frame for the rough opening height. 

  • Measure the distance vertically across your frame from top to bottom on the left, middle, and right side of the window from head jamb to the window sill.
  • Circle the smallest measurement of the three figures, this is your rough opening height.


* Tip: If your window sill is sloped, measure from the highest point of the window sill, which is usually right against your window.


Step 3. Measure the depth of the window’s opening. Screen Shot 2018-05-08 at 4.34.24 PM

  • Finding the correct depth is important because some windows will be too deep to fit into a replacement window opening.
  • Run a tape measure from the outside of the frame to the inside and take the measurement from edge to edge of the window frame.
  • Do this in several locations around the window frame and rely on the smallest measurement once again as your final dimension.

* Tip: If you can not open your window, measure the depth on each side of the glass and add them together. Then add the thickness of your window glass pane. A single window glass pane is usually 1/8″ or 3mm in thickness.

Learn about the standard window sizes you can expect to find here.

Measuring Rough Openings

It is vital that you get your window measurements exactly right. Otherwise, your home will be subject to drafts, which will increase your energy bills, expose you to the elements, and reduce your comfort at home.

  1. First, take a horizontal measurement of the replacement window frame. To do this, measure the distance across your frame at the top, at the middle, and at the bottom. Write down the smallest measurement of the three figures. The smallest measurement is the one to reference when purchasing a window. Do the same thing when measuring the height. Take a measurement at the center of the window and at either side to see which section is the smallest. Keep the two smallest numbers, one for the width and one for height and rely on those to make your purchasing decisions.
  2. You also have to measure the depth of the window opening. The depth is important because some windows will be too deep to fit into an opening. Run a tape measure from the outside of the frame to the inside and take the measurement from edge to edge. Do this in several different locations around the window and rely on the smallest figure.

Now that you have the measurements, you can use them to purchase windows that will fit into the space available. The windows should be approximately two inches narrower and two inches shorter than the measured space. This leaves an inch of space around each side of the window for positioning and adjusting the window to fit exactly where you want it. Use the depth measurement to ensure that your windows will not be sticking out of the frame opening—the windows don’t have to be much smaller than the depth. It may be best to find a local window replacement contractor.

What to Know Before Purchasing

Bring your rough opening window measurements with you and use them when purchasing your replacement windows and make sure you are paying a fair price and know the average replacement window installation costs.

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Take Me There

Most likely, you will be given the measurements of any windows that you plan on purchasing. Still, you should measure them yourself to ensure that they are the right size for your window openings.

  • Use a tape measure to measure the height of the window from top to bottom in three different places. Make note of the largest of the three measurements just in case the window is not uniform in height.
  • Then measure across the window from outside jamb to outside jamb to see how wide the window is. Take three measurements and use the largest.
  • Finally, measure the depth of the window unit in several locations to see if it is going to fit in your opening.
  • Compare the window measurements to the space available in your rough opening to see if the windows are compatible.

Most of the time, you will not have to do this when ordering windows because the measurements will be readily available. It is still good practice to take window measurements whenever you have windows physically available to you, even if it is just to double check someone else’s work. There is nothing worse than getting windows home that don’t fit.

A contractor measures a window

Easy Window Installation

Windows that are sized properly are much easier to install than windows that aren’t the right size. It’s a simple matter to put wedges around the window and level and plumb it before nailing, screwing, or stapling it into place no matter which window type you have installed. But when the space around the windows is not right, the installation will be more complicated. Either there will not be enough space to level and plumb the window and material will have to be cut away or the window will sit crooked. If there is too much space, it will be hard to shim the windows and lock them into place properly.

Draft-Free Windows

Drafts are common when windows are not properly installed. They occur for a few different reasons. The first reason is that there are gaps left open around the outside of the windows. This usually happens when there is too much space around a window and the gaps are not filled properly. It is much more difficult to fill in the space when it is larger than an inch.

Drafts are also caused when windows warp and the seals between panes break.This occurs when windows are crammed into too tight of a space and they expand and contract against the surrounding material.

The good news is that once you have learned how to measure windows, drafts will be a thing of the past.

Stable Windows

The windows will need some space between the glass and the frame to prevent issues due to expansion and contraction. The trick is that they need to be close enough to be effectively anchored to the frame. About an inch of space around the windows is the perfect compromise. It is enough space to keep the windows from pressing against the frame and causing problems, and close enough to facilitate secure anchoring.

Consider Exterior Space

If you opt for a swinging window—such as a casement or awning window—consider how much exterior space is available. These windows swing out from your home, so if you try to install one of them at the peak of your roof where overhangs get in the way, it will not work out well. If there is a possible obstruction around the outside of your window opening, make sure you account for it. Whether your home is built right next to the garage, or you have other parts of the home or yard to work around, keep in mind how much swing space you will need to allow.

Cutting a Rough Opening for a Replacement Window

What if you are more interested in making an opening around a window that you already purchased?

To do this, simply take the measurements of the window using the steps above and decide what depth, width, and height you need for the window. The depth can be the same as the window measurements, but the height and width of the opening both need to be two inches larger than the window to account for one inch of space on all sides. This leaves you with space to make adjustments, which will help keep the window level and plumb during installation.

With the opening cut to the proper specifications, you will have to build a frame for the window to sit in. First you will put in a header, and then you will put in cripple studs both above and underneath the rough opening for your window. This will help transfer the weight of your home down through the studs rather than through the window.

It is crucial to get the header and cripple studs in place before trying to install a window in the rough opening. Without these, you could put pressure on the window and cause it to warp and possibly break.

A contractor leveling a window

Hiring a Local Window Installer

You can attempt to install replacement windows on your own, but there are a lot of things that can potentially go wrong. If you are unsure about any of the steps, or if you don’t have construction experience, it is best to leave the work up to a professional. Sure, you will spend a bit more money, but you will save time in the process and benefit from a window warranty  when a licensed professional handles the installation. Best of all, you will have peace of mind that your windows will keep your home safe, protected, and comfortable for years to come.